Board Thread:Pokémon Help/@comment-32113832-20170526175622/@comment-27248965-20170527070642

In X/Y and OR/AS, EXP were not divided among Pokemon. All Pokemon participated get the full EXP, while those in party, but didn't participate, will get half if the EXP Share is ON (which, also has been turned into Key Item from X/Y onwards and now works like EXP All).

I heard that in Sun/Moon, the EXP system has been sort of reverted to how it works back in Black, White, B2 and W2, where the EXP gain would be scaled according to the level difference, but I'm not really sure because I haven't played Sun/Moon. You can check the page on Bulbapedia for reference, though.

Given that PBB will eventually be updated from OR/AS to Sun/Moon settings, I believe the EXP system will also be modified along to be on par with Sun/Moon. It's just a matter of time.